The differentiation of a B cell to a plasma cell represents one of the most dramatic changes in cellular architecture known. The massive increase in the secretory pathway is necessary to allow the plasma cell to become a factory dedicated to the synthesis, assembly and transport of immunoglobulin (Ig) molecules. The production of mature antibodies is aided and monitored by a group of resident ER proteins known as molecular chaperones. The ER concentration of these proteins is regulated by the unfolded protein response (UPR) pathway to accommodate cellular demands for the production of secretory pathway proteins. Recent studies revealed that the UPR is activated during plasma cell differentiation, and that the XBP-1 transcription factor, which is a component of this pathway, is essential for the production of plasma cells. It has been well-documented that the ER chaperone BiP binds to unassembled Ig heavy chains and prevents their premature transport. The release of BiP from unfolded substrates is a tightly controlled process that is regulated by ATP. We have identified three novel proteins that regulate BiP's ATPase activity and are therefore likely to be involved in the release of BiP from Ig substrates. Two of these, ERdj3 and ERdj4, are DnaJ homologues and increase BiP's rate of ATP hydrolysis and should serve to stabilize BiP's binding to unfolded Ig heavy chain substrates. The third protein, BAP, is the only known nucleotide exchange factor for BiP and should enhance the release of BiP from substrates. All three proteins are up-regulated during B cell differentiation. Both ERdj3 and ERdj4 are targets of the XBP-1 transcription factor, and ERdj3 is bound directly to unassembled Ig heavy chains. Thus, we hypothesize these BiP regulators may be required for terminal differentiation of B cells. Lastly, we have found that the ER-localized Herp protein, which contains a ubiquitin-like domain and may be involved in targeting ER proteins for degradation, is up-regulated by the UPR and during plasma cell differentiation. We hypothesize that these different proteins work together in a carefully orchestrated fashion to aid Ig assembly, monitor the success of this operation, and finally to target improperly folded or assembled Ig subunits for degradation. We propose a combination of biochemical, cell culture, and genetic experiments to determine the roles and requirements of the various BiP regulators in controlling Ig synthesis and the differentiation of B cells to plasma cells.